The prerequisites to unmount file systems are:
You must be superuser.
A file system must be available for unmounting. You cannot umount a file system that is busy. A file system is considered busy if a user is in a directory in the file system, or if a program has a file open in that file system. You can make a file system available for unmounting by:
Changing to a directory in a different file system.
Logging out of the system.
Using the fuser command to list all processes accessing the file system and to stop them if necessary. See "How to Stop All Processes for a File System" for more details.
Notify users if you need to unmount a file system they are using.